1. Field of the Invention
This invention relates to an electrolytic precision drilling method for incorporating a plurality of in-line holes having skewed (anti-parallel) axes and, more particularly, this invention relates to a method for incorporating cooling air holes in the trailing edge of a three-dimsionally twisted turbine blade by means of tubular electrodes through which a stream of electrolyte is directed on to the workpiece comprising an electrolyte manifold head to which all tubular electrodes are attached, and a guidance part disposed rigid with the workpiece, and provided with a coaxial bore to be formed in the blade, and including a feeding device for the tubular electrodes.
2. Description of the Prior Art
At the combustion gas temperatures encountered in modern applications the leading and trailing edges of turbine nozzle vanes and rotor blades need intensive cooling with air. This is achieved by blowing cooling air through a number of fine holes normally arranged in rows. These holes are formed using various known electrolytic precision drilling processes (also see CH-PS No. 457,798 or DT-OS No. 2,135,207). These known methods, however, permit of incorporating only a small number of holes (maximally some five to seven) at one pass because of the twist of the turbine blades, and of the resulting skewed axes of the holes. In the present state of the art of the practice is to relocate and rotate the workpiece or especially the turbine blade, after forming a number of holes in the proper location in a subsequent pass. The main disadvantage inherent in this method is that in the manufacture of turbine blades, which require from about 50 to 70 cooling air holes, the machining time and consequently all resulting negative or disadvantageous consequences of a manufacturing process are unduly extended.
In U.S. Pat. No. 3,803,015 there is disclosed a device for electrolytic precision drilling of a plurality of non-parallel bores, having a plurality of tubular electrodes attached to a single electrolyte manifold head, the latter being pushed upon drilling through a rigid guidance portion, a guidance bore being provided for each respective tubular electrode upon drilling. It is a main disadvantage of this known device or the method therefor, that the tubular electrodes are subjected to a considerable bending stress, due to the fact that they are also subjected to a double bending occurring during each bending process, the latter including an intermediate bending point, since the electrodes are forcibly guided in the guidance means. It is a further disadvantage of these forcible guidance means that increased friction results between the tubular electrodes and the walls of these forcible guidance means. Finally, the bending angle of these tubular electrodes is particularly large, since they are firmly gripped in the electrolyte manifold head, thus preventing any independent bending movement, the manifold head constituting the feeding means.